1. Field of the Invention
The present invention relates to a focused ion beam (FIB) equipment and an FIB processing method using this equipment, and in particular, to FIB equipment and an FIB processing method using this equipment to perform FIB processing of an LSI device having planarized fine multilayer wiring.
2. Description of the Related Art
Previous FIB equipment has been used for, for example, cutting away a prescribed position of an LSI device, to expose a cross-section in order to take scanning electron microscope (SEM) photographs. In such applications, the position of the FIB processing extends over a comparatively broad area, and therefore the precision required for detection of the processing position was not particularly high.
However, in recent years FIB equipment has come to be used to perform partial modification of wiring patterns in LSI devices and other processed samples in an apparatus with high vacuum, by irradiating the wiring pattern with a focused Ga ion beam to perform sputter etching for breaking wiring, or to form new wiring patterns by chemical vapor deposition (CVD) (using an organometallic gas containing tungsten) to connect wires. In order to thus partially break or connect wiring patterns in finely-structured LSI devices, the processing position must be detected with high precision, and FIB irradiation is performed.
On the other hand, planarization of multilayer wiring is generally employed in order to realize still more finely-structured LSI devices. In conventional multilayer wiring, problems occur due to wire-break failures caused by decreased coverage of metallization layers and wire breakage due to electromigration at steps, as step heights increase. Hence, in recent years, by planarizing the interlaminar insulation layers covering wiring patterns, using chemical-mechanical polishing (CMP) methods, and by forming wiring patterns on top, planarization of multilayer wiring structures has been performed.
It is extremely difficult to detect processing positions in such an LSI planarized structure with high precision. In observation of the surface of an LSI device inserted into FIB equipment, the function of a scanning ion microscope (SIM) using the FIB of the equipment is employed, scanning the LSI surface with the beam and detecting the reflected secondary electrons. It is only possible to detect processing positions on the surface by relying on the LSI surface shape detected in this manner. Hence, when no depressions or protrusions exist at the processing position, the processing position cannot be identified from a SIM image.
It has been proposed that the depressions and protrusions of bonding pads normally provided at the edges of a chip be detected by SIM, and that by moving the stage, targeting the coordinates of the processing position, the processing position be detected; but in this case, a wiring pattern position for processing cannot be accurately identified, as a result of stage errors accompanying stage movement.
Further, an image overlay method and a CAD navigation method have been proposed as methods for determining a processing position in FIB processing using conventional FIB equipment. In the image overlay method, an image of the processing position is acquired in advance with an optical microscope; the sample is transported into the FIB equipment; the sample is transported to the processing position coordinates through stage driving; a SIM image there obtained is superposed onto the optical microscope image by adjusting the magnification and rotation; the processing position is detected by relying on the superposed optical microscope image; and FIB processing is performed. The optical microscope can detect not only the surface shape, but also the shape of the lower-layer A1 wiring pattern, and so this method is effective for detection of wiring pattern positions in areas where the surface is planarized. In superposing the SIM image and the optical microscope image, images corresponding to both the depression and protrusions pattern existing on the sample LSI surface are used.
In the CAD navigation method, the stage coordinates of the FIB equipment are linked with the CAD data which is the pattern data of the LSI design data, and by specifying the processing position on the CAD data, the stage automatically moves to the sample processing position. In the above linking process, the CAD data image is superposed onto the SIM image based on bonding pads provided at the edges of the LSI, or on other prescribed patterns.
In the above image overlay method, it is necessary that a topmost-layer protrusion/depression pattern exist near the processing position on the LSI surface. Hence, when no surface protrusion/depression pattern exists over a broad area, the optical microscope image cannot be superposed on the SIM image in the processing area, and so a processing position cannot be detected within this area. In the CAD navigation method, there occurs position shifts resulting from stage precision errors accompanying movement of the stage from the positions of bonding pads provided at the chip edges to a processing position in the chip interior, and in such cases it is difficult to accurately detect the processing position. Moreover, in the CAD navigation method it is also ultimately necessary that the SIM image be superposed on the CAD data image based on a protrusion/depression pattern in the vicinity of the processing position, and so accurate detection of the processing position is difficult in a planarized area with few protrusion/depression patterns.
One object of the present invention is to provide FIB equipment, and an FIB processing method using same, which can accurately detect an FIB processing position on a planarized sample surface.
Another object of the present invention is to provide an LSI device formation method enabling the accurate detection of FIB processing positions in FIB equipment, and an FIB processing method using same.
In order to achieve the above objects, in a first aspect of this invention, FIB equipment, which irradiates a sample placed on a stage with a focused ion beam (FIB) to perform etching or pattern formation at the irradiation position, comprises an alignment mark formation unit to form an alignment mark by irradiating a periphery of a processing position with the FIB; and a processing position detection unit to superpose an optical microscope image of the area of the processing position at which the alignment mark is formed, and a scanning ion microscope image (SIM image) acquired by FIB irradiation, based on the alignment mark image, and to detect the processing position according to the superposed images.
In the above FIB equipment, even if the surface of the sample to be processed is planarized, a function of the FIB equipment for etching on pattern formation in extremely small areas can be used to automatically form the alignment mark on the planarized surface. Hence, an alignment mark is formed in the area of the processing position, an optical microscope image and SIM image of the area in which the alignment mark is formed are superposed based on the alignment mark, the processing position can be detected with high precision, and such position can be irradiated with an FIB. The alignment mark may be a depression-shape pattern etched by FIB processing, or may be a protrusion-shape pattern formed by FIB processing.
A preferred embodiment of the above invention specifies the processing position and the magnification of the processing area to the automatic alignment mark formation unit, alignment marks are formed in the periphery of the processing position with a size and at an interval which are nearly inversely proportional to the processing magnification. When the processing magnification is low, the shape of the alignment pattern is large, and intervals therebetween are also large; when the processing magnification is high, the alignment pattern shape and intervals therebetween are small in proportion. According to this, alignment marks can be formed on the periphery of the processing area from which a SIM image is acquired, without covering and hiding the pattern for processing.
In order to achieve the above object, in a second aspect of this invention, alignment marks are formed over an entire surface of a covering insulating layer at an uppermost layer of the sample LSI device for processing, at a size and interval chosen according to a design rule. Processing position detection is performed for this LSI device within the FIB equipment. In the processing position detection, a SIM image of the area including the processing position is superposed with an optical microscope image or CAD image, based on the above alignment marks, the processing position is detected, and the position is irradiated with an FIB.
Even if a sample LSI device for processing has a multilayer wiring structure which has been planarized, by forming alignment marks over the entire surface of the silicon oxide film, silicon nitride film, or other insulating cover film which is the uppermost layer, with size and interval chosen according to the design rule, alignment marks can always be made to exist in the vicinity of the processing position. These alignment marks can be formed simultaneously with the process in which etching removal is performed in the bonding pad area of the insulating cover film of the uppermost layer, or can be formed in an additional etching process thereto. To achieve this, the alignment marks data is added to the CAD data.
Hence, SIM images can be superposed on CAD data images as well as on electron microscope images, relying on the alignment marks. If images can be superposed in the processing area, then the superposed electron microscope image or CAD data image can be used to accurately detect the processing position even when the surface is flat, and FIB irradiation and processing can be performed.